When Arsenal scraped to victory over Crystal Palace on the opening day of the 2014-15 Premier League season, we learned a very important lesson about the Gunners.

Recent silverware combined with intelligent spending in the summer transfer window has outlined Arsene Wenger's side’s credentials as serious title contenders, but on Saturday’s evidence, the need to invest still remains.

That's not to say that Arsenal didn't look an impressive outfit—far from it—but they're just a few players away from completing their world-class puzzle. Here, we take a look at the candidates for doing just that before the transfer window slams shut.

Kostas Manolas

Michel Euler/Associated Press

Wenger has been busy bolstering his defence this summer, bringing in Mathieu Debuchy and Calum Chambers, but it seems that his work to the back line isn't done just yet.

Former club captain Thomas Vermaelen’s departure to Barcelona suddenly means there's a void to be filled at centre-back at the Emirates, and Wenger has reportedly turned his attention to Greece in a bid to do just that.

As Kevin Palmer of ESPN revealed, versatile Olympiakos defender Kostas Manolas is top of the Frenchman’s wish list, and the club have reportedly enquired about his availability.

Ann Heisenfelt/Associated Press

While there are some that won't know a great deal about Manolas, anyone who's watched Olympiakos or the Greek national team utilising their stubborn, defensive style has seen the young master at work.

Blessed with strength, pace, aerial prowess and composure on the ball, the 23-year-old is the ideal replacement for Vermaelen in defence. Goal.com’s Wayne Veysey says the Gunners want to wrap up the deal as soon as possible:

Arsenal working hard on deal for Kostas Manolas. Could look to wrap it up quickly.

Wrapping him up won't come at that hefty a price, either, as Olympiakos' sporting adviser Christian Karembeu recently said that as little as £8 million will see the Greek champions let their star go—per Jamie Sanderson of Metro: “In the last few days, interesting proposals have arrived from England. We are not willing to negotiate his sale for less than £8million. If we receive the right offer, we are willing to sell him.”

Abelimages/Getty Images

Manolas certainly fits the Arsenal transfer bill as a young star with ample room for progression, but he'd help the team on more than just that level.

With the Greek international adding to the club's choices at centre-back, Chambers can step into defensive midfield and ease fans' minds there.

It's difficult to argue against Arsenal making a swift move for Manolas on tactical, professional and financial fronts, and Wenger needs to dip into his pockets once again.

Adrien Rabiot

Michel Euler/Associated Press

Another player said to be on Wenger's radar is Paris Saint-Germain's Adrien Rabiot—a creative central midfielder who can both hold and push forward when required.

Rabiot has been linked with several big clubs around Europe this summer after impressing in Ligue 1 last season, but it seems that Arsenal are in pole position to sign the youngster.

Francisco Seco/Associated Press

As RMC revealed, via Metro, Arsenal have been given permission by PSG to hold talks with the 19-year-old as they look to bring in a new option in defensive midfield.

While able to play in an advanced role in midfield, Rabiot's brilliance is working defensively, and the statistics are there to back that up—per OptaJean:

At such a tender age, the Frenchman just needs a little bit of expert guidance before becoming the world-class midfielder that his potential threatens, and the Emirates is the best place to go about that.

However, as ESPN's Johnathan Johnson revealed, the Gunners will face competition for Rabiot's signature:

Per @DiMarzio Juventus had representatives at PSG-Bastia today to watch Ezequiel Lavezzi. May also have been there to discuss Adrien Rabiot.

It means that Wenger must act quickly on his mission to get the young starlet in red, and with a reported price tag of just £5.6 million, it looks a transfer worth chasing.

While landing the likes of Sami Khedira or William Carvalho would give Gunners fans peace of mind in the middle of the park, bringing a young talent through like Rabiot means that long-term success is a real possibility for Arsenal.